Two template pages/queries extremely slow

from technical perspective our museum website with your theme is getting closer to be most optimized for now. Thanks to your help getting bugs fixed!

But there are 2 nasty things left: there are two kind of “pages/posts/queries” (/?exhibition=…. and /?event=….) that are extremely slower (1,29sec to 14,45sec) than opening a WordPress post or page on the same site (un-cached of course, after caching they are equal).